A particularly advantageous application of the invention lies in igniting the pyrotechnical system of an "air bag" type device, but it may also be applied to missiles or space vehicles, and in general, to any electro-pyrotechnical initiation system that is required to have a high degree of operating regularity and a very high degree of reliability.
At present, one such initiator is known in which the resistive element is a small diameter cylindrical filament constituted by an alloy of high resistivity. The resistive filament is bonded at its respective ends to two current feeds.
In addition, initiators of the prior art include a thermosensitive substance that is in close contact with the resistive element. This substance is in the either of a mass of powder that has been compacted under high pressure, or else in the form of a "bead" type match head obtained by a conventional dipping technique. A very short length of time after electrical current begins to flow through the resistive element, the Joule effect causes the temperature of said thermosensitive substance to rise to its point of rapid self-decomposition, thereby leading to deflagration, and in an igniter said pyrotechnical reaction serves to ignite an auxiliary powder or a reinforcing pyrotechnical composition, or in a detonator it initiates detonation of a primary charge.
However, known initiators as described above suffer from a certain number of drawbacks:
firstly, handling and directly bonding the resistive filament having a diameter of a few tens of microns to the current feeds is always difficult to perform on an industrial scale;
in addition, the resulting initiators have electrical resistances that are rather widely dispersed because of the necessarily inaccurate definition of the working lengths between the bonds of different filaments;
and also, when using a thermosensitive substance in powder form, it can be difficult to achieve and to maintain the necessary close contact with the resistive filament. It is then necessary to use very strong metal rings at very high levels of compression. With a match head obtained by dipping, the shape of the bead and thus its mass are variable. In addition, in an igniter, the reinforcing powder is generally black powder, and to obtain proper igniting, it is necessary to have quite a large mass of thermosensitive substance (about 30 mg), and that requires weighing operations to be integrated in the manufacturing process.
Consequently, prior art initiators generally have a rather wide dispersion of performance characteristics with respect to sensitivity, operating time, and intensity of the resulting pyrotechnical phenomenon. In addition, they can give rise to problems under severe environmental conditions, in particular the shocks and vibrations that may be encountered in the lifetime of the product.